This invention relates to an A/D converter, and to techniques especially effective when utilized, e.g., in oversampling type A/D converters included in subscriber-line transmission equipment disposed in the subscriber circuit of an integrated digital communications network.
Echo cancellers are disposed in the subscriber circuits of an integrated digital communications network or the like. In addition, oversampling type A/D converters are included in echo cancellers.
As shown in FIG. 6, by way of example, the oversampling type A/D converter includes a delta summation type oversampling A/D conversion circuit A/D, which operates in accordance with an oversampling clock signal .phi.os, and a rate changing filter FIR, which integrates the output signals of the oversampling A/D conversion circuit A/D for fixed divisions of the frequency of the integrated signal in accordance with a sampling clock signal .phi.o so as to form output digital signals DO-Di of (i+1) bits.
The oversampling type A/D converter is described in, for example, "ISSCC '86, DIGEST OF TECHNICAL PAPERS" published in February 1986, pp. 180-181.
The present invention is usable in the environments of "A Voice Band Codec With Digital Filtering", IEEE TRANSACTIONS ON COMMUNICATIONS, Vol. COM-29, No. 6, June 1981, pp. 815-830, and also usable in "A Line Terminating Circuit Using the DSP Technique", Norio Tomaki et al, GLOBECOM '87, pp. 1731-1735.